Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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FXUS63 KDTX 210801
AFDDTX

Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
401 AM EDT Fri Sep 21 2018

.DISCUSSION...

Dynamic mid level wave now noted on waver vapor lifting across
Minnesota will accelerate east-northeast over the next 24 hours,
reaching eastern Quebec by Saturday morning. This trajectory will
direct the associated deepening surface low into central Ontario,
while sweeping the attendant strong cold front across the lower
peninsula today. Solid kinematic response well underway with this
system, prompting an increase and downstream expansion of stronger
winds within the burgeoning height fall center. Moist/mild pre-
frontal environment now entrenched locally, governed by increasing
deep layer southwest flow. Onset of daytime heating and ensuing
growth to the boundary layer will yield increasingly gusty pre-
frontal conditions this morning. The midday frontal timing will
leave a modest window for diurnal destabilization - MLCAPE in excess
of 1000 J/KG assuming a temperature/dewpoint of 82/69. This will
bring a limited opportunity for a broken line of low topped
convection to emerge along the advancing cold frontal slope,
centered 16z-19z early this afternoon. Greater potential exists
southeast of a Howell to Flint to Bad Axe line. Any development
raises concern for the possibility of strong wind gusts mixing down,
with 50 kt winds residing at 800mb. The ensuing cold air advection
timed favorably to capitalize on peak diurnal turbulent mixing will
then maintain gusty post-frontal westerly winds throughout the
latter half of the day. Outside of any convection, model guidance
uniformly support wind gusts today in the 35 to 40 mph range,
strongest winds with northward extent. Recent HRRR/RAP output and
ensemble probabilistic wind guidance lend moderate confidence for
gusts to reach near wind advisory criteria at times across the Tri-
Cities and Thumb corridor. In collaboration with surrounding
offices, went ahead with an advisory for this corridor.

Winds ease through the evening as the gradient relaxes and diurnal
mixing ceases. A period of stronger cold/dry air advection commences
during time. The degree of low level drying will dictate the extent
of low stratus that persists overnight. A moderate northerly flow
will persist tonight immediately downstream of building high
pressure. This will limit the potential for greater radiational
cooling, despite dewpoints plummeting into the lower and middle 40s.
Lows Saturday morning generally bottoming out in the 45 to 50 degree
range.

Dry and cooler conditions will hold through the upcoming weekend, as
high pressure gradually transits the region. Coldest conditions
noted Saturday, as 850 mb temperatures hover in the single digits.
This will favor highs of lower and middle 60s...or roughly 5-10
degrees below normal. Gradual airmass modification as upper heights
slowly climb will bring daytime highs back closer to normal on
Sunday.

&&

.MARINE...

A strengthening low pressure system will exit Lake Superior into
Ontario this morning. This low will pull a cold front through the
Great Lakes today which will result in a window of showers and
possibly thunderstorms. In addition, a tight pressure gradient and
strong wind field very low in the column will combine to produce
gale force winds today into tonight across the Lake Huron basin. The
day will begin with low end southwesterly gales ahead of the front
but as northwest flow ushers in colder air behind the front early
this afternoon, winds will increase to around 40 to 45 knots.
Farther away from the center of the low, Lakes St Clair and Erie
will likely stay below gales today thus the small craft advisories
will continue. The caveat would be any brief higher gust along or
just behind the front. High pressure will build in quickly behind
this system by Saturday, bringing improved marine conditions for the
weekend.

&&

.PREV DISCUSSION...
Issued at 1213 AM EDT Fri Sep 21 2018

AVIATION...

Dry weather is now anticipated for the TAF sites the remainder of
the night. Geopotential heights will rise overnight in response to
cyclogenesis over the northern Plains. Low level jet axis is in the
process of lifting overhead, per KDTX VAD wind, with environmental
wind suggesting greater than 40 knots of wind as low as 2000 ft agl.
Gradient wind tonight and stable warm advection will limit clouds
tonight although some lower potential exists for near surface haze.
Daytime heating will increase mixing depths and lead to southwest
winds of 25-35 knots by late morning. Latest forecast trends suggest
some higher gust potential in the Saginaw Valley at KMBS. Main item
to watch will be surface cold front and enhanced region of boundary
layer convergence west to east across the forecast area. This
boundary is expected to lead to low topped convection, more likely
showers between 16-19Z. Latest trends have sped up this potential.

For DTW...Quiet weather tonight with no cig concerns although vsby
restrictions possible. LLWS will remain problematic with 40 knots
down to 2000 ft agl. Forecast priority is southwest sustained/gusts
Friday in well mixed environment. Potential exists to exceed the
crosswind threshold. Second forecast priority is timing of surface
cold front that will carry a short duration low topped
shower/thunderstorm risk 16-20Z. Actual threat will likely be less
than 3 hours. Potential for higher wind gusts invof convection.

DTW THRESHOLD PROBABILITIES...

*  Low for ceilings 5000 ft or less tonight, moderate after 12Z
   Friday.

*  Moderate for thunderstorms impacting terminal in the 16z-19z time
   frame.

*  Moderate for crosswind threshold exceedance 240-260 degrees 30-35
   knots after 16z Friday.


&&

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Wind Advisory from 10 AM this morning to 6 PM EDT this evening for
     MIZ047>049-053>055.

     Beach Hazards Statement from 8 AM EDT this morning through this
     evening for MIZ049-055.

Lake Huron...Gale Warning from 8 AM this morning to midnight EDT tonight for
     LHZ361>363-421-422-441>443-462>464.

Lake St Clair...Small Craft Advisory from 8 AM this morning to midnight EDT tonight
     for LCZ460.

Michigan waters of Lake Erie...Small Craft Advisory from 8 AM this morning to midnight EDT tonight
     for LEZ444.

&&

$$

DISCUSSION...MR
MARINE.......DRK
AVIATION....CB


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